**Core Concept**
The demographic transition is a model that describes the changes in population growth rates and structures that occur in a country as it develops economically. It consists of five stages, each characterized by a distinct demographic profile. The demographic dividend refers to the economic benefits that arise when a country experiences a shift from a high to a low birth rate and death rate, leading to a surplus of working-age individuals.

**Why the Correct Answer is Right**
During the demographic transition, the demographic dividend is primarily due to a decrease in mortality rates, which leads to an increase in the working-age population. This increase in the working-age population is known as the "youth bulge." As the population ages and the birth rate decreases, the working-age population eventually peaks and begins to decline, leading to a decrease in the demographic dividend. This decrease in the demographic dividend can have significant economic implications, including a decrease in the labor force and an increase in the burden on the pension system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because an increase in the birth rate would actually lead to an increase in the demographic dividend, not a decrease.

**Option B:** This option is incorrect because an increase in the death rate would actually lead to a decrease in the working-age population, which would decrease the demographic dividend, but it is not the primary reason for the decrease.

**Option C:** This option is incorrect because an increase in the retirement age would actually lead to an increase in the working-age population, which would increase the demographic dividend, not decrease it.

**Clinical Pearl / High-Yield Fact**
A country typically experiences a demographic dividend when its total fertility rate (TFR) falls below the replacement rate of 2.1 children per woman. This decline in fertility rates leads to a shift in the population's age structure, resulting in a larger working-age population and a smaller dependent population.

**Correct Answer:** C.